Rechargeable LiFePO₄ Cells Using N-Methyl-N-butyl pyrrolidinium bis(trifluoromethane sulfonyl)imide-LiTFSI Electrolyte Incorporating Polymer Additives

We have incorporated polymer additives such as poly(ethylene glycol) dimethyl ether (PEGDME) and tetra(ethylene glycol) dimethyl ether (TEGDME) into N-methyl-N-butylpyrrolidinium bis(trifluoromethane sulfonyl)imide (PYR₁₄TFSI)-LiTFSI mixtures. The resulting PYR₁₄TFSI + LiTFSI + polymer additive ternary electrolyte exhibited relatively high ionic conductivity as well as remarkably low viscosity over a wide temperature range compared to the PYR₁₄TFSI + LiTFSI binary electrolytes. The charge/discharge cyclability of Li/LiFePO₄ cells containing the ternary electrolytes was investigated. We found that Li/PYR₁₄TFSI + LiTFSI + PEGDME (or TEGDME)/LiFePO₄ cells containing the two different polymer additives showed very similar discharge capacity behavior, with very stable cyclability at room temperature (RT). Li/PYR₁₄TFSI + LiTFSI + TEGDME/LiFePO₄ cells can deliver about 127 mAh/g of LiFePO₄ (74.7% of theoretical capacity) at 0.054 mA/cm² (0.2C rate) at RT and about 108 mAh/g of LiFePO₄ (63.4% of theoretical capacity) at 0.023 mA/cm² (0.1C rate) at −1 °C for the first discharge. The cell exhibited a capacity fading rate of approximately 0.09-0.15% per cycle over 50 cycles at RT. Consequently, the PYR₁₄TFSI + LiTFSI + polymer additive ternary mixture is a promising electrolyte for cells using lithium metal electrodes such as the Li/LiFePO₄ cell reported here. These cells showed the capability of operating over a significant temperature range (~0-~30 °C).